Test items for testing a device under test (DUT) 100 which is a GMSK modulation communication device include measurement of phase differences between an DUT output and an ideal signal. The DUT output is an analog base band wave forms I(t) and Q(t), which represent an in phase signal and a quadrature phase signal, respectively.
The DUT 100 receives transmission data TX.sub.dat of, for example, 270.833 kbps transmission speed and provides the received data a digital conversion process of a Gaussian filter characteristics by a digital signal processing technology installed therein. The Gaussian converted data is DA (digital-analog) converted, and thus, an analog base band waveforms I(t) and Q(t) are produced by the DUT 100.
An example of conventional measurement system for testing the DUT is shown in FIG. 3. The base band waveforms I(t) and Q(t) are modulated by a rectangular modulator 110 whereby the base band waveforms are rectangularly modulated with the use of a carrier signal f.sub.c having a several MHz carrier frequency. The modulated high frequency signal 120.sub.rf is measured by the measurement system as described below.
The measurement system is formed of an AD (analog-digital) converter 82, a buffer memory 83, an IQ demodulator 84, a phase/amplitude calculation part 86, and an error calculation part 90.
The AD converter 82, in receiving the high frequency signal 120.sub.rf which has been rectangularly modulated, samples the signal 120.sub.rf with a sampling clock f.sub.smp and converts the sampled data to a digital signal. The digitized signal for a certain period is stored in the buffer memory 83.
The IQ demodulator 84 extracts the base band I and Q signals which are received by the phase/amplitude calculation part 86. Amplitude data train 88.sub.amp and phase data train 87.sub.phase are obtained by the phase/amplitude calculation part 86 which are then provided to the error calculation part 90.
The error calculation part 90 is formed of a differential/IF removal part 92, a zero cross detection/compensation part 93, a clock phase/period detection part 94, a pro-synchronization bit pattern extraction part 95, an ideal data generator 96 and a difference detection/linear regression calculation part 97.
The differential/IF removal part 92, in receiving the phase data train 87.sub.phase noted above, differentiates the phase data train to convert to frequency data train. The zero cross detection/compensation part 93 receives the frequency data train and establishes timing reproduction points through a zero crossing method. Based on the timing reproduction points, the clock phase/period detection part 94 reproduces a baud rate clock of 270 kbps transmission speed through a least square method.
In the pro-synchronization bit pattern extraction part 95, in receiving the data from the zero cross detection/compensation part 93 and the amplitude data train 88.sub.amp, produces a bit pattern train which is synchronized with the actual data by using the baud rate clock reproduced in the foregoing.
The ideal data generator 96, in receiving the bit pattern train from the pro-synchronization bit pattern extraction part 95, generates ideal data having ideal phase points. The ideal data generated by the ideal data generator 96 is to be used as reference data which is provided to the difference detection/linear regression calculation part 97.
The difference detection/linear regression calculation part 97, in receiving the ideal data noted above and the phase data train 87.sub.phase from the phase/amplitude calculation part 86, calculates the difference between the actual data and the ideal data. Then the difference detection/linear regression calculation part 97 calculates an rms (root mean square) phase error and a frequency error through a regression process.
Because the conventional technology involves the above noted measurement and calculation means, in the difference detection/linear regression calculation part 97, when calculating the phase difference between the ideal data and the phase data train 87.sub.phase from the phase/amplitude calculation part 86, phase offsets in both of the data must be taken into consideration. Further, there is a disadvantage in that it is necessary to adjust the zero crossing points in the zero cross detection/compensation part 93. Further disadvantage is that offsets of the base band waveforms I(t) and Q(t) are unknown.
It is an object of the present invention to provide a measurement/calculation means which is not required to consider the phase offset between the reproduced ideal data and the phase data train to be measured.